Fluorescent lamp circuit employing a cathode follower configuration



March l1, 1969 FOLLOWER CONFIGURATION Filed March Armen/rr A United States Patent 3,432,723 FLUORESCENT LAMP CIRCUIT EMPLOYING A CATHODE FOLLOWER CONFIGURATION Jack V. Miller, 700 N. Auburn Ave., Sierra Madre, Calif. 91024, and Ronald W. Froelich, 444 Duarte Road, Arcadia, Calif. 91006 Filed Mar. 8, 1967, Ser. No. 621,563 U.S. Cl. 315--94 16 Claims Int. Cl. Hb 41/14.

ABSTRACT OF THE DISCLOSURE A highly eicient oscillator is energized by a battery. A voltage dividing capacitor and a fluorescent lamp are connected in series across the output of the oscillator. The value of the capacitor is selected with the operating frequency of the oscillator and the impedance of the lamp in mind. The capacitor preferably has a negative temperature tcoeicient. An ON-OFF switch includes provision for temporarily increasing the current through the lamp while it starts. In one embodiment, one of the lamp filaments forms part of the bias circuit for the oscillator.

Background of the invention This invention relates to electrical circuit arrangements and, more particularly, to electrical circuitry for eflciently energizing a fluorescent lamp.

Much effort has been devoted to developing a portable light source that is sufficiently long lasting, compact, light weight, and low priced to appeal to ordinary noncommercial users such as Sportsmen, campers, and home owners. A11 the above-mentioned characteristics of portable light sources depend in part on the elliciency of conversion from electrical energy, which is supplied by a battery, into light energy, which is radiated from a lamp. In actuality, a relatively small amount of light energy is required to illuminate an area the size of an average room. Much more energy is dissipated in thel form of heat during the energy lconversion from electricity to light.

In an attempt to achieve a more eicient conversion to light energy in portable light sources, prior art devices have resorted to fluorescent lamps. A D.C.-to-A.C. inverter and a step up transformer are sometimes employed to generate from a low-voltage battery the high voltage necessary to operate the fluorescent lamp. A capacitor is generally connected in parallel with the lamp, either to correct load power factor, to create a resonant circuit for driving the lamp load, or to reduce electrical noise from the lamp.

A problem associated with the efficient use of a fluorescent lamp is that the D.C.to-A.C. inverter must generate a higher voltage to start the lamp, i.e., to ignite it initially, than is thereafter necessary to maintain the operation of the lamp. If the higher starting voltage is maintained after the start is completed, then the inverter consumes more power than is actually necessary. To facilitate starting and thus reduce this voltage discrepancy, filaments are heated to aid initial ionization of the gas in the lamp. However, filament heating also represents an unwarranted drain on the battery because it is unnecessary once the lamp is started. Accordingly, to avoid the continual drain on the battery during operation, one prior art energizing circuit provides switches in series with the filament and the battery that disconnect the filament after the lamp is started. This arrangement adds to the cost of the light source, consumes more space, and calls on the user to remember to operate the filament switches at the appropriate time.

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Summary of the invention The invention contemplates the use of a very efficient oscillator to convert a low direct-current voltage into high alternating-current voltage for application to a fluorescent lamp. The active element of the oscillator is a transistor, preferably connected to operate as an emitter follower. A transformer is provided that has a magnetization curve substantially linear within the entire region of operation of the oscillator. The transformer has a winding divided into two sections by an intermediate tap. One section is connected between one terminal of the directcurrent source and the output terminal of the transistor. The other section is connected in series with a capacitor to form a regenerative feedback path between the output and input terminals of the transistor. The common terminal of the transistor is directly connected to the other terminal of the direct-current source. Finally a bias resistor is coupled between the input and common terminals of the transistor.

In addition a voltage dividing capacitor is connected in series with the lamp. The lamp is not started by resorting to filament heating. Instead, the oscillator is designed to produce an alternating-current voltage of sufficient magnitude to start the lamp independently of filament heating. The value of the capacitor is selected with respect t0 the operating frequency of the oscillator and the impedance of the lamp such that before the lamp starts the major portion of the voltage generated by the oscillator is impressed across the lamp and after the lamp starts the major portion of the voltage generated by the oscillator is impressed across the capacitor. Consequently, sufficiently high voltage is initially impressed across the terminals of the lamp to start it. Thereafter, a much smaller voltage is impressed across the terminals of the lamp, the remaining voltage appearing across the capacitor without appreciable power consumption.

According to a feature of the invention, the voltage dividing capacitor has a capacitance that varies inversely as a function of the R.M.S. current through it. Therefore, the current flowing through the lamp is regulated as the direct-current source voltage varies, with the result that little noticeable change in the illuminating effect of the lamp takes place over a wide range of source voltage.

According to another feature of the invention, an ON- OFF switch is provided for energizing the oscillator. In one position, the ON-OFF switch is spring loaded. One or more normally open contacts become closed when the ON-OFF switch is moved into the spring-loaded position. This contact completes a circuit that temporarily pro duces a higher starting current for the lamp while the user is holding the switch in the spring-loaded position.

Another feature of the invention, which is particularly advantageous when no ON-OFF switch is provided, contemplates the use of one of the lamp laments as part of the biasing circuit for the oscillator. As a result, part of the heat produced by the resistance that establishes the bias voltage for the oscillator is utilized in the lamp to start it; but no extra drain on the source results from this partial filament heating.

Brief description of the drawing The features of several specific embodiments of the invention are illustrated in the drawing, in which:

FIG. 1 is a schematic circuit diagram of lamp energizing circuitry employing an ON-OFF switch; and

FIG. 2 is a schematic circuit diagram of lamp energizing circuitry having no ON-OFF switch.

Description of specific embodiments In FIG. 1, a source of low direct-current voltage, represented by a battery 1, energizes an oscillator 2. Oscillator 2 converts the direct-current energy of battery 1 into a high alternating-current voltage that is impressed across a fluorescent lamp 3. A HI-LO switch 4 and an ON-OFF switch 5 control the energization of lamp 3. Oscillator 2 has as its active element a transistor 6 with a. collector connected to the negative terminal of battery 1. The collector of transistor 6 is the common terminal of the active element. In other words, transistor 6 operates as an emitter follower. A bias resistor 7 is connected between the base and collector of transistor 6. A transformer 9 is provided having a primary winding divided by a center tap into two sections, namely, sections and 16. A capacitor 8 is connected in series with section 16 between the vbase and emitter of transistor 6. Section 15 is connected between contacts 10 and 11 of ON-OFF switch 5 and the emitter of transistor 6. The positive terminal of battery 1 is connected to a slider arm 12 of ON-OFF switch 5, which rests on a contact 13 when switch 5 is deactuated and either on contact 10 or 11 when switch 5 is actuated. Thus, contact 13 is normally closed, and contacts 10 and 11 are normally open. A capacitor 14 is connected between contacts 10 and 11 and the negative terminal of battery 1 so it is across battery 1 when switch S is actuated. Capacitor 14 serves to reduce the fluctuation in circuit load impedance connected to battery 1 during a portion of the cycle of oscillator 2. The base of transistor 6 is coupled to a slider arm 20 of switch 5 by a resistor 21. A diode 22 is coupled between a contact 23 and capacitor 8. When ON-OFF switch 5 is deactivated, slider arm 20 rests on a contact 24 and when ON-OFF switch 5 is actuated, slider arm rests on either a contact 25 or contact 23. Thus, contact 24 is normally closed, and contacts 23 and 25 are normally open.

A capacitor is connected between one end terminal of the secondary winding of transformer 9 and a filament 31 of lamp 3. A filament 32 of lamp 3 is directly connected to the other end terminal of the secondary winding of transformer 9. Capacitor 30 and lamp 3 lfunction as a voltage divider. As connected, filaments 31 and 32 act solely as electrodes, each being essentially at equal potential all along its surface. Thus, filaments 31 and 32 impose no appreciable drain on battery 1. HI-LO switch 4 has a slider arm 33 that rests on a contact 35 when high illumination is desired and rests on a contact 34 when low illumination is desired. Contact 34- is normally closed, and contact 35 is normally open. A capacitor 36 is coupled between slider arm 33 and filament 31, while contact 35 is connected to the junction of the secondary winding of trans-former 9 and capacitor 30. When slider arm 33 rests on contact 35, the resultant capacitance in series with lamp 3 is decreased. Capacitors 30 and 36 are disc ceramics and have a negative temperature coefficient. Exemplary component values and types are as follows: battery 1-12 volts; capacitor 14-100 microfarads; capacitor 80.27 microfarad; capacitor 30- 0.0010 microfarad; capacitor 36-0.0015 microfarad; resistor 7--1500 ohms; resistor 21-68 ohms; diode 22Gene ral Electric A13F2; transistor 6-2N555; lamp 3-Sylvania F4T5/CW; transformer 9: section 15-31 turns, section 16-19 turns, secondary-1300 turns.

The core of transformer 9 is a linear low-loss material such as a molybdenum Permalloy and is designed to operate within the linear region of its magnetization curve during the entire cycle of oscillator 2. Thus, a relatively small amount of core heat loss results.

Slider arms 12 and 20 are ganged together. In the position wherein slider arms 12 and 20 are resting on contacts 11 and 23, respectively, ON-OFF switch 5 is spring loaded. Therefore, it remains in such position only as long as the user holds the switch there. After the user releases the switch, it springs back to the position wherein slider arms 12 and 20 are resting on contacts 10 and 25, respectively.

Slider arm 33 is ganged to move together with slider arms 12 and 20 such that when switch 5 is initially actuated and placed in the spring-loaded position, contacts 35, 11, and 23 are all closed. The user holds ON-OFF switch 5 in this position until lamp 3 starts. After lamp 3 is started, the user releases ON-OFF switch 5 and slider arms 12 and 20 spring back to rest on contacts 10 and 25, respectively. The latter condition is then maintained until switch 5 is again deactuated. Slider arm 33 is able to move independently of slider arms 12 and 20 when switch 5 is released from the spring-loaded position. Accordingly, contact 35 can be closed independently of the operation of switch 5, after the lamp starting period. Switches that perform the function described for switches 4 and 5 are commercially available.

The mode of operation of oscillator 2 will now Abe considered. For the purposes of this discussion, it is assumed that voltage has just been applied to the circuit shown in FIG. 1. In such case, capacitor 14 charges immediately to battery voltage and remains at battery voltage until the circuit is later shut off, serving to lower the A.C. impedance of the battery as seen by the load and to smooth out the oscillator current pulsations as seen by the battery.

When battery potential is first applied to the circuit, transistor 6 is initially biased in the forward direction as an emitter follower amplifier. Current flows between the collector and the emitter of transistor 6 because of the bias established by resistor 7.

As a result of the initial current flow through transistor 6 current also flows through section 15 thereby causing a voltage drop across section 15 in the `direction of the emitter of transistor 6. The current through section 15 also causes a flux in the core of transformer 9. As this flux increases a voltage drop is induced across section 16 in the direction of the base of transistor 6. This phase relationship between the voltage across sections 15 and 16 is often referred to in the art as an in-phase voltage relationship. The voltage induced across section 16 is ap plied to the base of transistor 6 through a capacitor 8 having a relatively low A.C. impedance. This causes transistor 6 to conduct more than the amount established initially by resistor 7. When transistor 6 conducts more, more current also flows through section 15. This further increase in current induces additional voltage across section 16, causing the transistor to conduct still more.

This regenerative process continues until transistor 6 is saturated, and any more increase in voltage at the base of the transistor 6 produces no further increase in voltage across section 15. Thus, the transistor has saturated, but the transformer continues to operate in the linear magnetization region.

When transistor 6 saturates and fails to provide further increases in ux in the transformer core, the voltage across section 16 starts to decrease slightly. This voltage decrease is coupled to the base of transistor 6, causing it to come out of saturation and its emitter voltage to rise slightly. This rise in voltage at the emitter of transistor 6 appears as a decreasing voltage drop across section 15, which in turn induces a further decreased voltage drop across section 16. Thus, transistor 6 conducts less. Again the regenerative process continues until transistor 6 is not only completely cut off, but a positive voltage has also been induced at the base of the transistor 6 nearly equal in magnitude to the negative voltage at the base when transistor 6 is saturated. After transistor 6 cuts off, the transformer action again causes the induced voltage to collapse with the result that transistor again moves into saturation. The net result is a circuit with well sustained oscillations occurring by the employment of a transistor in a common-collector configuration with in-phase voltage gain, in conjunctoin with a transformer forming in-phase voltage feed-back. The build-up and collapse of flux in the core induces a voltage across the secondary of transformer 9.

Capacitors 30 and 36 are Selected to provide the desired current for a given impedance of lamp 3 and frequency of oscillation. For the exemplary components given above the frequency is approximately 5.6 kilocycles per second. When the circuit is initially energized, the lamp draws negligible current before it lights, and the voltage drop is negligible across capacitor 30. Thus, essentially all of the voltage induced across the secondary of the transformer (about 550 Volts for the exemplary values given) appears across the electrodes of the lamp. The high voltage across the lamp electrodes is sufficient to cause adequate electrostatic field gradient within the lamp, and cause arc current between electrodes to increase. As arc electrode current increases, a voltage drop now occurs across capacitor 30, which in turn reduces the voltage impressed on the lamps electrodes. When suicient arc current density within the lamp has been established, and the lamp electrode surfaces are adequately heated, normal gas ionization within the lamp is established. This in turn excites the fluorescent powders along the bulbs inner Walls to produce visible white light. In normal operation, the voltage drop across the lamp is about 60 volts, after the lamp is started, and is relatively independent of current through the lamp due to the inherently negative dynamic resistance characteristics of the lamps discharge arc. After the lamp is started the major portion of voltage developed by the transformer secondary winding appears across capacitor 30. Capacitor 30 has very little resistance associated with it, so little power is dissipated by large R.M.S. voltages impressed across it during operation of the lamp.

In the interest of battery conservation, FIGURE 1 shows an embodiment wherein the total value of series capacitance is contained in two capacitors 30 and 36. For normal operation and lamp starting, capacitors 30 and 36 are connected in parallel by switch 4. For battery conservation, slider arm 33 of switch 4 is moved to rest on contact 34, leaving only capacitor 30 in series with the lamp.

The reduction in series capacitance of the lamp circuit by this action results in a reduction of about 40% in lamp current, although the change in Voltage across the lamps electrodes is negligible. This decrease in lamp current is seen at the battery as a comparable reduction in battery current drain.

The starting of lamp 3 is further aided by the placement of resistor 21 and diode 22 in parallel with capacitor 8 during the starting period when switch 5 connects wipers 12 and 20 to contacts 11 and 23 respectively. The diode and resistor simulate the lamps load on the transformer during negative voltage swings at the transformer, before the arc has been formed in the lamp. This simulated load allows capacitor 8 to reach the proper operating bias level more rapidly, which in turn causes the oscillator to achieve proper operating conditions more rapidly.

As mentioned above, capacitors 30 and 36 have negative temperature coefficients. Thus, the capacitance varies inversely with the temperature. Due to resistive dielectric loss effects, the temperature of capacitors 30 and 36 is related to the R.M.S. current passing through them. Therefore, their capacitance varies inversely with R.M.S. current. As the voltage across the terminals of battery 1 decreases in the course of use, the current passing through capacitor 30 and/or 36 also tends to decrease. Consequently, the temperature vof the capacitors tends to decrease and capacitance increases. Since the reactive impedance of the capacitive element of the voltage divider decreases, a larger current flows through lamp 3. This compensates for the drop in the magnitude of the voltage appearing across the secondary winding of transformer 9. Consequently, as the battery terminal voltage varies, little change in the illuminating characteristics of lamp 3 takes place. In summary, capacitors 30 and 36 regulate the current flow through lamp 3. For variations in terminal voltage of battery 1 between 6 and 12 volts, the illumination obtained from lamp 3 changes by about only 20 percent.

In the circuit arrangement of FIG. 2, all the elements with the exception of a resistor 37 and capacitor 38 have a counterpart in FIG. 1. In this arrangement, resistor 37 is connected in series with filament 32 between the base and the collector of transistor 6. The combined resistance of resistor 37 and filament 32 equals the resistance of resistor 7 in FIG. l. The capacitance of capacitor 38 is equal to the capacitance of capacitors 30 and 36 in parallel. Filament 32 serves the dual purpose of preheating one electrode in lamp 3 so it starts more easily and providing a resistance across which a portion of the battery voltage appears for establishment of a transistor bias voltage. This arrangement is particularly useful where for some reason an ON-OFF switch of the nature of that shown in FIG. 1 is not desired. It could, for example, be easily incorporated in a unit for use with a screw socket to which a source of low directcurrent voltage is supplied.

What is claimed is:

1. A fluorescent lamp energizing circuit comprising:

a source of D C. voltage;

a regenerative oscillator including a low loss transformer and a single transistor arranged to be driven to saturation during each cycle of operation of the oscillator, the transistor being connected in an emitter follower configuration between the D.C. source and the transformer, the transformer being arranged such that it has a linear magnetization curve within the region of operation of the oscillator;

a primary winding divided into a first and a second section and a secondary Winding;

Voltage dividing means connected in series with the secondary winding of the transformer; and

a liuorescent lamp connected in series with the voltage dividing means and the secondary of the transformer whereby energization of the oscillator causes a major portion of the voltage across the secondary winding of the transformer to be imposed Iacross the electrodes of the fluorescent lamp during starting and across the Voltage dividing means after starting.

2. An energizing circuit according to claim 1 wherein the voltage dividing means is a capacitor, the capacit-ance of which is a value determined by the frequency of the oscillator and the impedance of the lamp.

3. An energizing circuit according to claim 2 in which the lamp has at least one filament, the ends of which areI connected together such that the filament acts solely as an electrode having an equal potential all along its surface.

4. An energizing circuit according to claim 3 wherein the lamp filaments are short circuited and the lamp is started without resort to filament heating.

5. An energizing circuit according to claim 1 wherein the transformer utilizes a molybdenum permalloy core for limiting core losses to a minimum.

6. The circuit arrangement of claim 2, in which the capacitor has a negative temperature coefficient and sufficient dielectric loss to cause current regulation.

7. The circuit arrangement of claim 6, in which an ON-OFF` switch is provided having first, second and third positions, the switch being spring loaded such that it remains in the third position only when held there and moves to the second position when released from the third position;

first circuit means is provided connecting the D.C. source, the transformer and the transistor to switch contacts corresponding to the second and third switch positions; and

second circuit means is provided connecting an auxiliary capacitor to contacts corresponding to the third switch position whereby the oscillator is energized by the source of D.C. voltage when the ON-OFF switch is in the second and third positions and the auxiliary capacitor is connected in parallel with the voltage dividing capacitor when the ON-OFF switch is in the third position.

8. The circuit arrangement of claim 7 in which a HI-LO switch is provided having first and second positions with the HI-LO switch being ganged to the ON-OFF switch such that while the ON-OFF switch is in its third position the HI-LO switch is in the second position `and when the HI-LO switch is released from its third position to its second position the HI-LO switch is movable between its first and second positions independently of the ON-OFF switch; and

third circuit means is provided connecting the auxiliary capacitor to contacts corresponding to the second position of the HI-LO switch whereby the auxiliary capacitor is connected in parallel with the voltage dividing capacitor while the HI-LO switch is in its second position.

9. The circuit arrangement of claim 8 in which a circuit for facilitating the start of the lamp is provided comprising a resistor and a diode in series; and

fourth circuit means is provided connecting the facilitating circuit and the transistor to contacts corresponding to the third position of the ON-OFF switch whereby the facilitating circuit is connected between said terminals of the transistor only while the ON-OFF switch is in said third position.

10. A circuit arrangement for energizing a fiuorescent lamp from a direct current source to produce continuous unblinking illumination from the lamp comprising:

a source of direct current voltage having first and second terminals;

an oscillator comprising a transistor having input,

output and common terminals and a low loss transformer having a substantially linear magnetization curve within the region of operation of the oscillator, the transformer having a primary and a secondary winding with the primary winding being divided into a first and a second section;

means for connecting the first section of the primary winding between the first terminal of the direct current source and the output terminal of the transistor;

a capacitor connected in series with the second section of the primary winding between the output and input terminals of the transistor to form a regenerative feedback path;

means for connecting the common terminal of the transistor to the second terminal of the direct current source;

a bias resistor connected between the input and common terminal of the transistor; and

a fluorescent lamp connected across the secondary of the transformer in series with a voltage dividing capacitor, one side of the fluorescent lamp being connected in common with the common terminal of the transistor.

11. A circuit arrangement according to claim 10 wherein the transistor is arranged to be driven to saturation during each cycle of the oscillator and the transformer is arranged such that the voltage across the first section of the primary winding is in phase with the voltage across the second section, the voltage across the second section of the primary winding being fed back to the base of the transistor to cause oscillations to be generated by the oscillator.

12. The circuit arrangement of claim 11, in which an 8 ON-OFF switch is provided having first, second and third positions, the switch being spring loaded such that it remains in the third position only when held there and moves to the second position when released from the third position;

first circuit means is provided connecting the D.C.

source, the transformer and the transistor to switch contacts corresponding to the second and third switch portions; and

second circuit means is provided connecting an auxiliary capacitor to contacts corresponding to the third switch position whereby the oscillator is energized by the source of D.C. voltage when the ON-OFF switch is in the second and third positions and the auxiliary capacitor is connected in parallel with the voltage dividing capacitor when the ON-OFF switch is in the third position.

13. The circuit arrangement of claim 12 in which a HI-LO switch is provided having first and second positions with the HI-LO switch being ganged to the ON-OFF switch such that while the ON-OFF switch is in its third position the HI-LO switch is in the second position and when the HI-LO switch is released from its third position to its second position the HI-LO switch is movable between its first and second positions independently of the ON-OFF switch; and

third circuit means is provided connecting the auxiliary capacitor to contacts corresponding to the second position of the HI-LO switch whereby the auxiliary capacitor is connected in parallel with the voltage dividing capacitor while the HI-LO switch is in its second position.

14. The circuit arrangement of claim 13 in which a circuit for facilitating the start of the lamp is provided comprising a resistor and a diode in series; and

fourth circuit means is provided connecting the facilitating circuit and the input and common terminals of the transistor to contacts corresponding to the third position of the ON-OFF switch whereby the facilitating circuit is connected between said terminals of the transistor only when the ON-OFF switch is in said third position.

15. The circuit arrangement of claim 10, in which the transistor is connected in the emitter follower configuration so its base is the input terminal, its emitter is the output terminal, and its collector is the common terminal; land the first and second sections of the transformer are formed by a single winding having an intermediate tap.

16. The circuit arrangement of claim 10, in which the lamp yhas at least one filament which is connected in series with the bias resistor Ibetween the input and common terminals of the transistor to establish in part the voltage bias for the oscillator.

References Cited UNITED STATES PATENTS 3,016,478 1/ 1962 Kadell. 3,247,422 4/ 1966 Schultz 315-206 3,257,607 6/1966 Pintell 323-93 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

U.S. Cl. X.R. 

